1. Field of the Invention
The present invention relates to a battery charger for charging a battery pack of an electric appliance such as an electric cleaner and a power driven tool, and more particularly to an improvement in a circuit configuration of the battery charger.
2. Description of the Prior Art
General circuit configuration of a conventional battery charger of constant current type is shown in FIG. 6. The circuit comprises of a primary DC power circuit 1 for rectifying and smoothing an AC power derived from a commercial AC power source (not shown) through AC terminals 1a, 1b, a field-effect transistor FET having an insulated gate and operating as a chopper for intermitting, at a predetermined frequency, a DC power supplied from the primary DC power circuit 1. The circuit also includes a transformer T for drop of voltage having a primary coil for receiving the intermitting DC power from the field-effect transistor FET and a secondary coil for outputting a voltage-dropped power of the intermitting waveform, a secondary DC power circuit 2 for rectifying and smoothing the voltage-dropped power, a current detector circuit 3 for detecting a charging current I, a secondary control circuit 4 for outputting a pulse signal (hereinafter called a charge control signal S2) the pulse width of which is modulated in such a manner that it becomes smaller when the charging current I is large and that it becomes larger when the charging current I is small, and a primary control circuit 5 for varying the duty cycle of an on-off gate signal G1 to be supplied to the field effect transistor FET.
A battery pack 6 comprises battery 6 to be charged and a thermostat THM connected in series with the battery B for detecting charging-up of the battery B. The battery pack 6 is mounted on a mounting portion formed with a body (not shown) of the battery charger for fixedly receiving the battery pack 6, so that terminals 6a, 6b and 6c of the battery pack 6 are electrically connected with a positive terminal 7a, a negative terminal 7b and a thermostat terminal 7c of the battery charger, respectively. Assuming that the battery B is normal, the battery B is initially charged at a very little current through a diode D1, a resistor R1 for limiting the current and a heat-sensitive protection element such as an OCR on a side of the battery charger. The current detector circuit 3 subsequently detects the charging current I based on the voltage across a shunt resistor R2 and outputs a current detector signal S1 to the secondary control circuit 4 which detects a charging voltage V. The secondary control circuit 4 excites its internal relay coil (not shown) so as to close a relay switch SW, so that the battery B may be charged on full-scale. At the same time with such full-scale charging, the exciting current of the relay coil of the secondary control circuit 4 flows through the terminal 7c (6c), the thermostat THM and the terminal 7b (6b). (In FIG. 6, the configuration of the secondary control circuit is partly omitted.)
The battery charger of this type is constructed to charge the battery B by supplying the charging current of constant value. When the analog value of the voltage of the current detector signal S1 from the current detector circuit 3 does not reach a predetermined value, a larger amount of the charging current is supplied to the secondary side. Thus, the secondary control circuit 4 produces the charge control signal S2, the pulse of which has been modulated to have a large width, and the charge control signal S2 is subsequently supplied to the primary control circuit 5 as an analog signal through CR integration circuit 8 of a PWM demodulation circuit and a photocoupler (optical isolator) PC1. (PWM means pulse width modulation.) The primary control circuit 5 produces the gate control signal G1 of relatively large duty cycle corresponding to the pulse with large width and supplies the same to the field-effect transistor FET. Then, the current flowing through the primary coil of the transformer T increases, so that the current flowing through the secondary coil also increases to the effect that more charging current is supplied to the battery B. This means the charging current of constant value is supplied to the battery B.
When the battery pack 6 having the battery B which is short circuited is mounted on the battery charger, an excessive current flows through the shunt resistor R2. This may be also caused when the battery B is defective. Such excessive current is detected by the current detector circuit 3 which supplies an excessive current detecting signal S3 to the primary control circuit 5 through a photocoupler PC2. The primary control circuit 5 modulates the gate control signal G1 to have a smaller duty cycle based on the excessive current detecting signal S3. The charging current of the secondary side therefore decreases, so that the supply of the excessive current to the battery B can be relieved.
In FIG. 6, R3 to R6 are resistors and C1 is a capacitor.
The prior art battery charger as described above has the following drawbacks:
The charge control signal S2 having a modulated pulse width and outputted from the secondary control circuit 4 is firstly converted to an analog signal S2' by the CR integration circuit 8 of the PWM demodulation circuit. The analog signal S2' is applied to a light emitting diode LED and therefore, it is drawn by an emitter of a phototransistor Tr1 of the photocoupler PC1. The photocoupler PC1 is used to provide different values between the primary earth voltage and the secondary earth voltage so as to attain electrical insulation. The photocoupler PC1 has, however, a transfer characteristic (such as hFE) which varies with photocouplers, and it has inferior temperature characteristic. Further, it considerably deteriorates with age. Therefore in manufacturing a circuit of a battery charger, it requires to select photocouplers which have the similar characteristics to each other and to make a fine control while remaining low reliability to the variation with age. Particularly, the emitter voltage of the phototransistor Tr1 as a light receiver is not linearly identical with the voltage of the analog signal S2'. Therefore, it is difficult to reliably transfer the analog signal S2', and the duty cycle of the gate control signal G1 from the primary control circuit 5 inevitably includes an error. Thus, the prior art has a problem in its control system.
Further, in the prior art charging circuit, for limiting the excessive charging current which may be caused on the secondary side, the duty cycle of the gate control signal to be supplied from the primary control circuit is compulsorily limited in the path from the current detector circuit 3 to the photocoupler PC2 which acts as an information transmitter from the secondary side to the primary side, so that the amount of supply of power to the secondary side can be decreased. However, in such prior art charging circuit, a transient limitation is very difficult when the current is abruptly increased to reach an excessive value. Further, since the path of transmission of the information is rather long, the response is notably delayed and it cannot meet with immediate current limitation. The battery charger is applied to charge various types of batteries, and there is some possibility to cause excessive heating when the excessive current has been produced during the charging operation.